Rapid, covalent addition of phosphine to dithiolene in a molybdenum tris(dithiolene). A new structural model for dimethyl sulfoxide reductase

Triphenylphosphine (PPh(3)) rapidly and reversibly adds to the bdt ligand in the molybdenum tris(dithiolene) complex Mo(tfd)(2)(bdt) [tfd = S(2)C(2)(CF(3))(2); bdt = S(2)C(6)H(4)], turning chelating bdt into the monodentate zwitterionic ligand SC(6)H(4)SPPh(3). A second PPh(3) molecule fills the newly created open site in the crystallographically characterized product Mo(tfd)(2)(SC(6)H(4)SPPh(3))(PPh(3)), which is a structural model for dimethyl sulfoxide (DMSO) reductase. While the complex is only a precatalyst for reduction of DMSO by PPh(3) (the initially low catalytic rate increases with time), Mo(tfd)(2)(SMe(2))(2) was found to be catalytically active without an induction period.     

Monodithiolene molybdenum(V, VI) complexes: a structural analogue of the oxidized active site of the sulfite oxidase enzyme family

The active sites of the xanthine oxidase and sulfite oxidase enzyme families contain one pterin-dithiolene cofactor ligand bound to a molybdenum atom. Consequently, monodithiolene molybdenum complexes have been sought by exploratory synthesis for structural and reactivity studies. Reaction of [MoO(S(2)C(2)Me(2))(2)](1-) or [MoO(bdt)(2)](1-) with PhSeCl results in removal of one dithiolate ligand and formation of [MoOCl(2)(S(2)C(2)Me(2))](1-) (1) or [MoOCl(2)(bdt)](1-) (2), which undergoes ligand substitution reactions to form other monodithiolene complexes [MoO(2-AdS)(2)(S(2)C(2)Me(2))](1-) (3), [MoO(SR)(2)(bdt)](1-) (R = 2-Ad (4), 2,4,6-Pr(i)(3)C(6)H(2) (5)), and [MoOCl(SC(6)H(2)-2,4,6-Pr(i)(3))(bdt)](1-) (6) (Ad = 2-adamantyl, bdt = benzene-1,2-dithiolate). These complexes have square pyramidal structures with apical oxo ligands, exhibit rhombic EPR spectra, and 3-5 are electrochemically reducible to Mo(IV)O species. Complexes 1-6 constitute the first examples of five-coordinate monodithiolene Mo(V)O complexes; 6 approaches the proposed structure of the high-pH form of sulfite oxidase. Treatment of [MoO(2)(OSiPh(3))(2)] with Li(2)(bdt) in THF affords [MoO(2)(OSiPh(3))(bdt)](1-) (8). Reaction of 8 with 2,4,6-Pr(i)(3)C(6)H(2)SH in acetonitrile gives [MoO(2)(SC(6)H(2)-2,4,6-Pr(i)(3))(bdt)](1-) (9, 55%). Complexes 8 and 9 are square pyramidal with apical and basal oxo ligands. With one dithiolene and one thiolate ligand of a square pyramidal Mo(VI)O(2)S(3) coordination unit, 9 closely resembles the oxidized sites in sulfite oxidase and assimilatory nitrate reductase as deduced from crystallography (sulfite oxidase) and Mo EXAFS. The complex is the first structural analogue of the active sites in fully oxidized members of the sulfite oxidase family. This work provides a starting point for the development of both structural and reactivity analogues of members of this family.     

Oxygen/sulfur substitution reactions of tetraoxometalates effected by electrophilic carbon and silicon reagents

Reactions of [MO(4)](2)(-) (M = Mo, W) with certain carbon and silicon electrophiles were investigated in acetonitrile in order to produce species of potential utility in the synthesis of analogues of the sites in the xanthine oxidoreductase enzyme family. Silylation of [MoO(4)](2)(-) affords [MoO(3)(OSiPh(3))](1)(-), which with Ph(3)SiSH is converted to [MoO(2)S(OSiPh(3))](1)(-). Reaction with (Ph(3)C)(PF(6))/HS(-) yields the tetrahedral monosulfido species [MO(3)S](2)(-), previously obtained only from the aqueous system [MO(4)](2)(-)/H(2)S. Dithiolene chelate rings are readily introduced upon reaction with 1,2-C(6)H(4)(SSiMe(3))(2), leading to the square pyramidal trioxo complexes [MO(3)(bdt)](2)(-), a previously unknown dithiolene molecular type. Further ring insertion occurs upon reaction of [WO(3)(bdt)](2)(-) with 1,2-C(6)H(4)(SSiMe(3))(2), giving [WO(2)(bdt)(2)](2)(-). Related reactions occur with [ReO(4)](1)(-). Treatment with 1 equiv of (Me(3)Si)(2)S produces [ReO(3)S](1)(-); with 3 equiv of 1,2-C(6)H(4)(SSiMe(3))(2), [ReO(bdt)(2)](1)(-) is obtained with concomitant Re(VII) --> Re(V) reduction. X-ray structures are reported for [MO(3)S](z)(-) (M = Mo, W, z = 2; M = Re, z = 1), [MO(3)(bdt)](2)(-), and [WO(2)(OSiPh(3))(bdt)](1)(-), a silylation product of [WO(3)(bdt)](2)(-). [MoO(3)(bdt)](2)(-) is related to the site of inactive sulfite oxidase, and [WO(2)(OSiPh(3))(bdt)](1)(-) should closely approximate the metric features of the [(dithiolene)MoO(2)(OH)] site in inactive aldehyde/xanthine oxidoreductase. This work provides convenient syntheses of known and new derivatives of tetraoxometalates, among which is entry to a unique class of oxo-monodithiolene complexes.     

Metal-dependent and redox-selective coordination behaviors of metalloligand [MoV(1,2-benzenedithiolato)3]- with CuI/AgI ions

The synthesis and characterization of two coordination polymers, {Cu(I)[Mo(V)(bdt)(3)]·0.5Et(2)O}(n) (1·0.5Et(2)O, bdt: o-benzenedithiolato) and {Ag(I)[Mo(V)(bdt)(3)]}(n) (2), composed of redox-active [Mo(V)(bdt)(3)](-) metalloligand with Cu(I) and Ag(I) ions are reported. The complexation reactions of [Mo(V)(bdt)(3)](-) with Cu(II)(ClO(4))(2) or Ag(I)ClO(4) commonly lead to the formation of one-dimensional (1-D) coordination polymers. The presence of Cu(I) in 1·0.5Et(2)O strongly indicates that the Cu(II) ion is reduced during the complexation reaction with [Mo(V)(bdt)(3)](-), which acts as an electron donor. The total dimensionalities of the assembled structures of 1·0.5Et(2)O and 2 are significantly different and related to the type of additional metal ions, Cu(I) and Ag(I). In contrast to the isolated 1-D chain structure of 1·0.5Et(2)O, complex 2 has a three-dimensional (3-D) assembled structure constructed from additional π-π stacking interactions between adjacent [Mo(V)(bdt)(3)](-) moieties. These structural differences influence the solubility of the complexes in organic solvents; complex 1·0.5Et(2)O is soluble as origomeric species in highly polar solvents, while 2 is insoluble in organic solvents and water. Coordination polymers 1·0.5Et(2)O and 2 were investigated by UV-vis spectroscopy in the solid state, and that in solution together with their electrochemical properties were also investigated for 1 because of its higher solubility in polar organic solvents. Complex 1·0.5Et(2)O dissolved in CH(3)CN demonstrates concentration-dependent UV-vis spectra supporting the presence of coordinative interactions between [Mo(V)(bdt)(3)](-) moieties and Cu(I) ions to create the origomeric species even in solutions, an observation that is supported also by electrochemical experiments.     

Probing the electronic structure of [MoOS(4)](-) centers using anionic photoelectron spectroscopy

Using photodetachment photoelectron spectroscopy (PES) in the gas phase, we investigated the electronic structure and chemical bonding of six anionic [Mo(V)O](3+) complexes, [MoOX(4)](-) (where X = Cl (1), SPh (2), and SPh-p-Cl (3)), [MoO(edt)(2)](-) (4), [MoO(bdt)(2)](-) (5), and [MoO(bdtCl(2))(2)](-) (6) (where edt = ethane-1,2-dithiolate, bdt = benzene-1,2-dithiolate, and bdtCl(2) = 3,6-dichlorobenzene-1,2-dithiolate). The gas-phase PES data revealed a wealth of new electronic structure information about the [Mo(V)O](3+) complexes. The energy separations between the highest occupied molecular orbital (HOMO) and HOMO-1 were observed to be dependent on the O-Mo-S-C(alpha) dihedral angles and ligand types, being relatively large for the monodentate ligands, 1.32 eV for Cl and 0.78 eV for SPh and SPhCl, compared to those of the bidentate dithiolate complexes, 0.47 eV for edt and 0.44 eV for bdt and bdtCl(2). The threshold PES feature in all six species is shown to have the same origin and is due to detaching the single unpaired electron in the HOMO, mainly of Mo 4d character. This result is consistent with previous theoretical calculations and is verified by comparison with the PES spectra of two d(0) complexes, [VO(bdt)(2)](-) and [VO(bdtCl(2))(2)](-). The observed PES features are interpreted on the basis of theoretical calculations and previous spectroscopic studies in the condensed phase.     

A new class of sulfido/oxo(dithiolene)-molybdenum(IV) complexes derived from sulfido/oxo-bis(tetrasulfido)molybdenum(IV) anions

Mono(dithiolene)sulfidomolybdenum(IV) complexes, [MoS(S4)(bdt)](2-) (2) and [MoS(S4)(bdtCl2)](2-) (3) (1,2-benzenedithiolate = bdt, 3,6-dichloro-1,2-benzenedithiolate = bdtCl2), were prepared by the substitution reaction of a tetrasulfido ligand in known [MoS(S4)2](2-) (1) with the corresponding dithiol. Complexes 2 and 3 were irreversibly oxidized to give bis(mu-sulfido) dimolybdenum(V) species, {[MoS(bdt)]2(mu-S)2}(2-) (4) and {[MoS(bdtCl2)]2(mu-S)2}(2-) (5), in aerobic acetonitrile. Mono(dithiolene)oxomolybdenum(IV) complexes, [MoO(S4)(bdt)](2-) (7) and [MoO(S4)(bdtCl2)](2-) (8), that are oxo derivatives of 2 and 3 were also synthesized from a known [MoO(S4)2](2-) (6) of an oxo derivative of 1 and the corresponding dithiol. Further, the electrophilic addition of dimethyl acetylenedicarboxylate to 7 gave [MoO(bdt)(S2C2(COOMe)2)](2-) (9), and ligand substitution of the tetrasulfido group of 7 with bdt and bdtCl2 yielded [MoO(bdt)2](2-) ( 10) and [MoO(bdt)(bdtCl2)](2-) (11), respectively. New sulfido/oxo molybdenum complexes were characterized by (1)H NMR, IR, ESI-MS, Raman, and UV-vis spectroscopies; cyclic voltammetry; and elemental analysis, and crystal structures of 2, 3, 5, 7, and 8 were determined by X-ray analysis.     

A biomimetic approach to oxidized sites in the xanthine oxidoreductase family: synthesis and stereochemistry of tungsten(VI) analogue complexes

Two series of square pyramidal (SP) monodithiolene complexes, [M (VI)O 3- n S n (bdt)] (2-) and their silylated derivatives [M (VI)O 2- n S n (OSiR 3)(bdt)] (-) ( n = 0, M = Mo or W; n = 1, 2, M = W), synthesized in this and previous work, constitute the basic molecules in a biomimetic approach to structural analogues of the oxidized sites in the xanthine oxidoreductase enzyme family. Benzene-1,2-dithiolate (bdt) simulates native pyranopterindithiolene chelation in the basal plane, tungsten instead of the native metal molybdenum was employed in sulfido complexes to avoid autoreduction, and silylation models protonation. The complexes [MO 3(bdt)] (2-) and [MO 2(OSiR 3)(bdt)] (-) represent inactive sites, while [MO 2S(bdt)] (2-) and [MOS(OSiR 3)(bdt)] (-), with basal sulfido and silyloxo ligands, are the first analogues of the catalytic sites. Also prepared were [MOS 2(bdt)] (2-) and [MS 2(OSiR 3)(bdt)] (-), with basal sulfido and silyloxo ligands. Complexes are described by angular parameters which reveal occasional distortions from idealized SP toward a trigonal bipyramidal (TBP) structure arising from crystal packing forces in crystalline Et 4N (+) salts. Miminized energy structures from DFT calculations are uniformly SP and reproduce experimental structures. For example, the correct structure is predicted for [WO 2S(bdt)] (2-), whose basal and apical sulfido diastereomers are potentially interconvertible through a low-lying TBP transition state for pseudorotation. The lowest energy tautomer of the protonated form is calculated to be [WOS(OH)(bdt)] (-), with basal sulfido and hydroxo ligands. Computational and experimental structures indicate that protein sites adopt intrinsic coordination geometries rather than those dictated by protein structure and environment.     

Monoanionic molybdenum and tungsten tris(dithiolene) complexes: a multifrequency EPR study

Numerous Mo and W tris(dithiolene) complexes in varying redox states have been prepared and representative examples characterized crystallographically: [M(S(2)C(2)R(2))(3)](z) [M = Mo, R = Ph, z = 0 (1) or 1- (2); M = W, R = Ph, z = 0 (4) or 1- (5); R = CN, z = 2-, M = Mo (3) or W (6)]. Changes in dithiolene C-S and C-C bond lengths for 1 versus 2 and 4 versus 5 are indicative of ligand reduction. Trigonal twist angles (Θ) and dithiolene fold angles (α) increase and decrease, respectively, for 2 versus 1, 5 versus 4. Cyclic voltammetry reveals generally two reversible couples corresponding to 0/1- and 1-/2- reductions. The electronic structures of monoanionic molybdenum tris(dithiolene) complexes have been analyzed by multifrequency (S-, X-, Q-band) EPR spectroscopy. Spin-Hamiltonian parameters afforded by spectral simulation for each complex demonstrate the existence of two distinctive electronic structure types. The first is [Mo(IV)((A)L(3)(5-?))](1-) ((A)L = olefinic dithiolene, type A), which has the unpaired electron restricted to the tris(dithiolene) unit and is characterized by isotropic g-values and small molybdenum superhyperfine coupling. The second is formulated as [Mo(V)((B)L(3)(6-))](1-) ((B)L = aromatic dithiolene, type B) with spectra distinguished by a prominent g-anisotropy and hyperfine coupling consistent with the (d(z(2)))(1) paramagnet. The electronic structure disparity is also manifested in their electronic absorption spectra. The compound [W(bdt)(3)](1-) exhibits spin-Hamiltonian parameters similar to those of [Mo(bdt)(3)](1-) and thus is formulated as [W(V)((B)L(3)(6-))](1-). The EPR spectra of [W((A)L(3))](1-) display spin-Hamiltonian parameters that suggest their electronic structure is best represented by two resonance forms {[W(IV)((A)L(3)(5-?))](1-) ? [W(V)((A)L(3)(6-))](1-)}. The contrast with the corresponding [Mo(IV)((A)L(3)(5-?))](1-) complexes highlights tungsten's preference for higher oxidation states.     

High-nuclearity sulfide-rich molybdenum[bond]iron[bond]sulfur clusters: reevaluation and extension

High-nuclearity Mo[bond]Fe[bond]S clusters are of interest as potential synthetic precursors to the MoFe(7)S(9) cofactor cluster of nitrogenase. In this context, the synthesis and properties of previously reported but sparsely described trinuclear [(edt)(2)M(2)FeS(6)](3-) (M = Mo (2), W (3)) and hexanuclear [(edt)(2)Mo(2)Fe(4)S(9)](4-) (4, edt = ethane-1,2-dithiolate; Zhang, Z.; et al. Kexue Tongbao 1987, 32, 1405) have been reexamined and extended. More accurate structures of 2-4 that confirm earlier findings have been determined. Detailed preparations (not previously available) are given for 2 and 3, whose structures exhibit the C(2) arrangement [[(edt)M(S)(mu(2)-S)(2)](2)Fe(III)](3-) with square pyramidal Mo(V) and tetrahedral Fe(III). Oxidation states follow from (57)Fe M?ssbauer parameters and an S = (3)/(2) ground state from the EPR spectrum. The assembly system 2/3FeCl(3)/3Li(2)S/nNaSEt in methanol/acetonitrile (n = 4) affords (R(4)N)(4)[4] (R = Et, Bu; 70-80%). The structure of 4 contains the [Mo(2)Fe(4)(mu(2)-S)(6)(mu(3)-S)(2)(mu(4)-S)](0) core, with the same bridging pattern as the [Fe(6)S(9)](2-) core of [Fe(6)S(9)(SR)(2)](4-) (1), in overall C(2v) symmetry. Cluster 4 supports a reversible three-member electron transfer series 4-/3-/2- with E(1/2) = -0.76 and -0.30 V in Me(2)SO. Oxidation of (Et(4)N)(4)[4] in DMF with 1 equiv of tropylium ion gives [(edt)(2)Mo(2)Fe(4)S(9)](3-) (5) isolated as (Et(4)N)(3)[5].2DMF (75%). Alternatively, the assembly system (n = 3) gives the oxidized cluster directly as (Bu(4)N)(3)[5] (53%). Treatment of 5 with 1 equiv of [Cp(2)Fe](1+) in DMF did not result in one-electron oxidation but instead produced heptanuclear [(edt)(2)Mo(2)Fe(5)S(11)](3-) (6), isolated as the Bu(4)N(+)salt (38%). Cluster 6 features the previously unknown core Mo(2)Fe(5)(mu(2)-S)(7)(mu(3)-S)(4) in molecular C(2) symmetry. In 4-6, the (edt)MoS(3) sites are distorted trigonal bipramidal and the FeS(4) sites are distorted tetrahedral with all sulfide ligands bridging. M?ssbauer spectroscopic data for 2 and 4-6 are reported; (mean) iron oxidation states increase in the order 4 < 5 approximately 1 < 6 approximately 2. Redox and spectroscopic data attributed earlier to clusters 2 and 4 are largely in disagreement with those determined in this work. The only iron and molybdenum[bond]iron clusters with the same sulfide content as the iron[bond]molybdenum cofactor of nitrogenase are [Fe(6)S(9)(SR)(2)](4-) and [(edt)(2)Mo(2)Fe(4)S(9)](3-)(,4-).     

Silylation, sulfidation, and benzene-1,2-dithiolate complexation reactions of oxo- and oxosulfidomolybdates(VI) and -tungstates(VI)

The synthesis and structures of two types of molecules are presented: [MVIO3 - nSn(OSiR2R')]1- (M = Mo, n = 0-3; M = W, n = 3) and [MVIO2(OSiR2R')(bdt)]1- (M = Mo, W; bdt = benzene-1,2-dithiolate). For both types, R2R' are Me3, Pri3, Ph3, Me2But and Ph2But. The complete series of oxo/sulfido/silyloxo molybdenum complexes has been prepared. Complexes with n = 0 are readily prepared by the silylation of Ag2MoO4 and sustain mono- or disulfidation with Ph3SiSH to form a species with n = 1 and n = 2, respectively. Complexes with n = 3 are accessible by the silylation of [MOS3]2-. Structures of the representative series members [MoO3(OSiPh2But)]1-, [MoO2S(OSiPh3)]1-, [MoOS2(OSiPri3)]1-, [MoS3(OSiPh2But)]1-, and also [WS3(OSiMe2But)]1-, all with tetrahedral stereochemistry, are presented. Benzene-1,2-dithiolate complexes are prepared by the reaction of [MoO3(OSiR2R')]1-with the dithiol or by the silylation of previously reported [MO3(bdt)]2-. The structures of [MoO2(OSiPh2But)(bdt)]1- and [WO2(OSiPri3)(bdt)]1- conform to square-pyramidal stereochemistry with an oxo ligand in the apical position. The role of these complexes in the preparation of site analogues of the xanthine oxidoreductase enzyme family is noted. The sulfidation reactions reported here point to the utility of Ph3SiSH and Pri3SiSH as reagents for MoVI-based oxo-for-sulfido conversions.     

Hexacyanometalate molecular chemistry: heptanuclear heterobimetallic complexes; control of the ground spin state

Following a bottom-up approach to nanomaterials, we present a rational synthetic route from hexacyanometalates [M(CN)(6)](3-) (M=Cr(III), Co(III)) cores to well-defined heptanuclear complexes. By changing the nature of the metallic cations and using a localised orbital model it is possible to control and to tune the ground state spin value. Thus, with M=Cr(III), d(3), S=3/2, three heptanuclear species were built and characterised by mass spectrometry in solution, by single-crystal X-ray diffraction and by powder magnetic susceptibility measurements, [Cr(III)(CNbondM'L(n))(6)](9+) (M'=Cu(II), Ni(II), Mn(II), L(n)=polydentate ligand), showing spin ground states S(G)=9/2 [Cu(II)], with ferromagnetic interactions J(Cr,Cu)=+45 cm(-1), S(G)=15/2 [Ni(II)] and J(Cr,Ni)=+17.3 cm(-1), S(G)=27/2 [Mn(II)], with an antiferromagnetic interaction J(Cr,Mn)=-9 cm(-1), (interaction Hamiltonian H=-J(Cr,M) [S(Cr)Sigma(i)S(M)(i)], i=1-6). With M=Co(III), d(6), S=0, the heptanuclear analogues [Co(III)(CN-M'L(n))(6)](9+) (M'=Cu(II), Ni(II), Mn(II)) were similarly synthesised and studied. They present a singlet ground state and allow us to evaluate the weak antiferromagnetic coupling constant between two next-nearest neighbours M'-Co-M'.     

Strong electronic interaction between two dimolybdenum units linked by a tetraazatetracene

The large rigid dianion fluoflavinate, C(14)H(8)N(4)(2)(-), consisting of four fused and planar six-membered rings with four nitrogen donor atoms, has been used to link two metal-to-metal bonded and redox-active Mo(2)(n)()(+) units which are each locally bridged by three additional groups, collectively denoted [Mo(2)]. In 1, the [Mo(2)] units are Mo(2)(DAniF)(3) (DAniF = N,N'-di-p-anisylformamidinate), and in 5, they are trans-Mo(2)(DAniF)(2)(O(2)CCH(3)) groups. These [Mo(2)](fluoflavinate)[Mo(2)] compounds show three reversible one-electron oxidation steps, one more than all other [Mo(2)](linker)[Mo(2)] species known to date. The first two redox processes are metal-based, and the third one has been assigned to a ligand oxidation by comparison to that of paddlewheel compound 4 which contains only one dimolybdenum unit with a monoanionic fluoflavinate ligand. Chemical oxidations of 1 produce the singly- and doubly-oxidized species 2 and 3, respectively. All compounds have been characterized by X-ray crystallography and, as appropriate, by various techniques such as NMR, EPR, near-IR, and UV-vis. The fluoflavinate ligand strongly mediates electronic communication between the dimetal units, and the mixed valence species 2 can be described as electronically delocalized. Calculations at the DFT level using a variety of functionals support such an assignment and indicate that a strong transition in the NIR for the singly oxidized species can be assigned to the HOMO-1 to SOMO transition.     

Interconversion and Reactivity of Two Heterometallic Tin-Containing Cuboidal Clusters from [Mo(3)S(4)(H(2)O)(9)](4+): X-ray Structure of the Single Cube with an Mo(3)SnS(4) Core

The Mo(3)SnS(4)(6+) single cube is obtained by direct addition of Sn(2+) to [Mo(3)S(4)(H(2)O)(9)](4+). UV-vis spectra of the product (0.13 mM) in 2.00 M HClO(4), Hpts, and HCl indicate a marked affinity of the Sn for Cl(-), with formation of the more strongly yellow [Mo(3)(SnCl(3))S(4)(H(2)O)(9)](3+) complex complete in as little as 0.050 M Cl(-). The X-ray crystal structure of (Me(2)NH(2))(6)[Mo(3)(SnCl(3))S(4)(NCS)(9)].0.5H(2)O has been determined and gives Mo-Mo (mean 2.730 ?) and Mo-Sn (mean 3.732 ?) distances, with a difference close to 1 ?. The red-purple double cube cation [Mo(6)SnS(8)(H(2)O)(18)](8+) is obtained by reacting Sn metal with [Mo(3)S(4)(H(2)O)(9)](4+). The double cube is also obtained in approximately 50% yield by BH(4)(-) reduction of a 1:1 mixture of [Mo(3)SnS(4)(H(2)O)(10)](6+) and [Mo(3)S(4)(H(2)O)(9)](4+). Conversely two-electron oxidation of [Mo(6)SnS(8)(H(2)O)(18)](8+) with [Co(dipic)(2)](-) or [Fe(H(2)O(6)](3+) gives the single cube [Mo(3)SnS(4)(H(2)O)(12)](6+) and [Mo(3)S(4)(H(2)O)(9)](4+) (up to 70% yield), followed by further two-electron oxidation to [Mo(3)S(4)(H(2)O)(9)](4+) and Sn(IV). The kinetics of the first stages have been studied using the stopped-flow method and give rate laws first order in [Mo(6)SnS(8)(H(2)O)(18)](8+) and the Co(III) or Fe(III) oxidant. The oxidation with [Co(dipic)(2)](-) has no [H(+)] dependence, [H(+)] = 0.50-2.00 M. With Fe(III) as oxidant, reaction steps involving [Fe(H(2)O)(6)](3+) and [Fe(H(2)O)(5)OH](2+) are implicated. At 25 degrees C and I = 2.00 M (Li(pts)) k(Co) is 14.9 M(-)(1) s(-)(1) and k(a) for the reaction of [Fe(H(2)O)(6)](3+) is 0.68 M(-)(1) s(-)(1) (both outer-sphere reactions). Reaction of Cu(2+) with the double but not the single cube is observed, yielding [Mo(3)CuS(4)(H(2)O)(10)](5+). A redox-controlled mechanism involving intermediate formation of Cu(+) and [Mo(3)S(4)(H(2)O)(9)](4+) accounts for the changes observed.     

Syntheses, characterizations and properties of [Mo2O2S2]-based oxothiomolybdenum wheels incorporating bisphosphonate ligands

We report the syntheses and characterizations of the first polyoxothiometalate complexes isolated from the reaction of the oxothiocationic [Mo(V)(2)O(2)S(2)](2+) precursor and bisphosphonate ligands H(2)O(3)PCR(OH)PO(3)H(2) (R = C(4)H(5)N(2), zoledronic acid; R = C(3)H(6)NH(2), alendronic acid). [(Mo(2)O(2)S(2)(H(2)O))(4)(O(3)PC(O)(C(4)H(6)N(2))PO(3))(4)](8-) (Mo(8)S(8)(Zol)(4)) and [(Mo(2)O(2)S(2)(H(2)O))(4)(O(3)PC(O)(C(3)H(6)NH(3))PO(3))(4)](8-) (Mo(8)S(8)(Ale)(4)) contain four Mo(V) dimers connected via bisphosphonate ligands. These compounds offer a unique opportunity to compare the structures and properties of cyclic compounds obtained with [Mo(2)O(2)S(2)](2+) and with [Mo(2)O(4)](2+). The oxothio compounds appear less stable in solution than the oxo analogue, confirming the higher lability and versatility of [Mo(2)O(2)S(2)]-based compounds compared to [Mo(2)O(4)]-based POMs. Multinuclear and multidimensional solid-state NMR studies were carried out to complement X-ray diffraction analysis. Information on short-range interactions, dynamic behaviors, and local disorder within the crystalline materials are therefore reported. Furthermore, the electrocatalytic properties of Mo(8)S(8)(Ale)(4) and of the analogous [(Mo(2)O(4)(H(2)O))(4)(O(3)PC(O)(C(3)H(6)NH(3))PO(3))(4)](8-) (Mo(8)O(8)(Ale)(4)) immobilized onto the surface of a glassy carbon electrode were studied, thus evidencing the ability of [Mo(2)O(2)S(2)]-based cycles to promote the reduction of protons into hydrogen, whereas the oxo analogue appeared inactive.     

Influence of the ligand on the coupling between the metal-based electrons in face-shared [M2X9]3- (M = Mo,W; X = F,Cl, Br,I) systems

Orbital overlap and spin polarization effects in Mo and W [M(2)X(9)](3)(-) halide and in [M(2)X'(3)X' '(6)](3)(-) mixed-halide systems have been investigated by means of density-functional calculations performed on the S = 0, S = 3, and reference states of these species. For the regular [M(2)X(9)](3)(-) systems, a strong linear correlation between the two factors has been obtained, and decreasing trends in both the overlap energy and the spin polarization energy upon descending the halide group have been observed. These trends can be related to the changes in the size and covalency of the ligands and in the nature of the metal-bridge interaction. For the mixed-ligand [M(2)X'(3)X' '(6)](3)(-) systems, important deviations (from the behavior of the regular systems), which are apparently the result of particular structural and energetic characteristics, have been observed.     

Experimental fingerprints for redox-active terpyridine in [Cr(tpy)2](PF6)n (n = 3-0), and the remarkable electronic structure of [Cr(tpy)2]1-

The molecular and electronic structures of the four members, [Cr(tpy)(2)](PF(6))(n) (n = 3-0; complexes 1-4; tpy = 2,2':6',2″-terpyridine), of the electron transfer series [Cr(tpy)(2)](n+) have been determined experimentally by single-crystal X-ray crystallography, by their electro- and magnetochemistry, and by the following spectroscopies: electronic absorption, X-ray absorption (XAS), and electron paramagnetic resonance (EPR). The monoanion of this series, [Cr(tpy)(2)](1-), has been prepared in situ by reduction with KC(8) and its EPR spectrum recorded. The structures of 2, 3, 4, 5, and 6, where the latter two compounds are the Mo and W analogues of neutral 4, have been determined at 100(2) K. The optimized geometries of 1-6 have been obtained from density functional theoretical (DFT) calculations using the B3LYP functional. The XAS and low-energy region of the electronic spectra have also been calculated using time-dependent (TD)-DFT. A consistent picture of the electronic structures of these octahedral complexes has been established. All one-electron transfer processes on going from 1 to 4 are ligand-based: 1 is [Cr(III)(tpy(0))(2)](PF(6))(3) (S = (3)/(2)), 2 is [Cr(III)(tpy(?))(tpy(0))](PF(6))(2) (S = 1), 3 is [Cr(III)(tpy(?))(2)](PF(6)) (S = (1)/(2)), and 4 is [Cr(III)(tpy(??))(tpy(?))](0) (S = 0), where (tpy(0)) is the neutral parent ligand, (tpy(?))(1-) represents its one-electron-reduced π radical monoanion, (tpy(2-))(2-) or (tpy(??))(2-) is the corresponding singlet or triplet dianion, and (tpy(3-))(3-) (S = (1)/(2)) is the trianion. The electronic structure of 2 cannot be described as [Cr(II)(tpy(0))(2)](PF(6))(2) (a low-spin Cr(II) (d(4); S = 1) complex). The geometrical features (C-C and C-N bond lengths) of these coordinated ligands have been elucidated computationally in the following hypothetical species: [Zn(II)Cl(2)(tpy(0))](0) (S = 0) (A), [Zn(II)(tpy(?))Cl(NH(3))](0) (S = (1)/(2)) (B), [Zn(II)(tpy(2-))(NH(3))(2)](0) (S = 0 or 1) (C), and [Al(III)(tpy(3-))(NH(3))(3)](0) (S = (1)/(2) and (3)/(2)) (D). The remarkable electronic structure of the monoanion has been calculated and experimentally verified by EPR spectroscopy to be [Cr(III)(tpy(2-))(tpy(??))](1-) (S = (1)/(2)), a complex in which the two dianionic tpy ligands differ only in the spin state. It has been clearly established that coordinated tpy ligands are redox-active and can exist in at least four oxidation levels.     

Direct measurement of the hydrogen-bonding effect on the intrinsic redox potentials of [4Fe-4S] cubane complexes

To probe how H-bonding effects the redox potential changes in Fe-S proteins, we produced and studied a series of gaseous cubane-type analogue complexes, [Fe(4)S(4)(SEt)(3)(SC(n)H(2n+1))](2-) and [Fe(4)S(4)(SEt)(3)(SC(n)H(2n)OH)](2-) (n = 4, 6, 11; Et = C(2)H(5)). Intrinsic redox potentials for the [Fe(4)S(4)](2+/3+) redox couple involved in these complexes were measured by photoelectron spectroscopy. The oxidation energies from [Fe(4)S(4)(SEt)(3)(SC(n)H(2n)OH)](2-) to [Fe(4)S(4)(SEt)(3)(SC(n)H(2n)OH)](-) were determined directly from the photoelectron spectra to be approximately 130 meV higher than those for the corresponding [Fe(4)S(4)(SEt)(3)(SC(n)H(2n+1))](2-) systems, because of the OH...S hydrogen bond in the former. Preliminary Monte Carlo and density functional calculations showed that the H-bonding takes place between the -OH group and the S on the terminal ligand in [Fe(4)S(4)(SEt)(3)(SC(6)H(12)OH)](2-). The current data provide a direct experimental measure of a net H-bonding effect on the redox potential of [Fe(4)S(4)] clusters without the perturbation of other environmental effects.     

Unusual Anions [LAl(SH)(S)]- and [LAl(S)2]2- stabilized by weakly coordinating imidazolium cations. synthesis of LAl(SSiMe2)2O (L = HC[C(Me)N(Ar)]2, Ar = 2,6-iPr2C6H3)

Deprotonation of an Al-SH moiety has been achieved easily by using N-heterocyclic carbene as the base. Monomeric mono- and bis-imidazolium salts [C(t)H(+)][LAl(SH)(S)](-) ([C(t)H(+)] = N,N'-bis-tert-butylimidazolium), [C(m)H(+)][LAl(SH)(S)](-), and [C(m)H(+)](2)[LAl(S)(2)](2-) ([C(m)H(+)] = N,N'-bismesitylimidazolium), containing unusual anions [LAl(SH)(S)](-) and [LAl(S)(2)](2-), have been synthesized in nearly quantitative yields. Furthermore, [C(m)H(+)](2)[LAl(S)(2)](2-) has been successfully used for the preparation of LAl(SSiMe(2))(2)O containing the [O(Me(2)SiS)(2)](2-) ligand.     

EPR Studies of Oxo-Molybdenum(V) Complexes with Sulfur Donor Ligands: Implications for the Molybdenum Center of Sulfite Oxidase

A series of six-coordinate compounds containing a chelating dithiolate coordinated to the [LMo(V)O](2+) unit (L = hydrotris(3,5-dimethyl-1-pyrazolyl)borate) have been characterized by EPR spectroscopy as models for the molybdenum centers of pterin-containing molybdenum enzymes. The structure of LMoO(bdt) (1) (bdt = 1,2-benzenedithiolate) has been determined by X-ray crystallography; the space group is P2(1)/n with a = 10.727(1) ?, b = 14.673(2) ?, c = 15.887(2) ?, beta = 100.317(4) degrees and Z = 4. Compound 1 exhibits distorted octahedral stereochemistry; the terminal oxo group and the sulfur atoms are mutually cis to one another. The Mo=O distance is 1.678(4) ?, and the average Mo-S distance is 2.373(2) ?. The EPR parameters for 1, determined from simulation of the frozen-solution spectrum, are g(1) = 2.004, g(2) = 1.972, g(3) = 1.934 and A(1)((95,97)Mo) = 50.0 x 10(-)(4), A(2) = 11.4 x 10(-)(4), A(3) = 49.7 x 10(-)(4) cm(-)(1). The EPR parameters for several LMo(V)O{S(CH(2))(x)()S} compounds (x = 2-4) with saturated chelate skeletons are similar to those of 1, indicating that it is the coordinated S atoms and not unsaturation of the chelate skeleton that gives rise to the large g values for 1. The presence of g components larger than the free-electron value is ascribed to low-energy charge transfer transitions from the filled sulfur pi orbitals to half-filled Mo d orbitals. The EPR spectrum of [LMo(V)O{S(2)P(OEt)(2)}](+) shows an unusually large isotropic (31)P hyperfine splitting of 66.1 x 10(-)(4) cm(-)(1) from the noncoordinated phosphorus atom. The frozen-solution EPR spectra of the low-pH and high-pH forms of sulfite oxidase have been reinvestigated in D(2)O and the anisotropic g and A((95,97)Mo) parameters determined by simulation of the spectrum arising from the naturally abundant Mo isotopes (75% I = 0, 25% I = (5)/(2)). The EPR parameters for the low-pH form are g(1) = 2.007, g(2) = 1.974, g(3) = 1.968 and A(1) = 56.7 x 10(-)(4), A(2) = 25.0 x 10(-)(4), A(3) = 16.7 x 10(-)(4) cm(-)(1). The EPR parameters for the high-pH form are g(1) = 1.990, g(2) = 1.966, g(3) = 1.954 and A(1) = 54.4 x 10(-)(4), A(2) = 21.0 x 10(-)(4), A(3) = 11.3 x 10(-)(4) cm(-)(1). These are the first determinations of the complete A((95,97)Mo) hyperfine components for an enzyme that possesses an [Mo(VI)O(2)](2+) core in its fully oxidized state.